A stepped automatic transmission comprises a planetary gear mechanism and a plurality of friction elements, and switches a gear position by switching an engagement state of each friction element such that a desired gear position is obtained.
It is known that when a release failure or an engagement failure occurs in a friction element in this type of automatic transmission, control is performed to specify the failed friction element in order to secure drivability.
For example, in a technique disclosed in JP11-280886A, characteristic data generated when a failure occurs in a friction element are stored in advance for each gear position, and on the basis of these stored data and an actual speed ratio, the failed friction element and the manner of the failure, i.e. whether the failure is a release failure or an engagement failure, are specified.